1. Field of the Invention
The present invention relates to a mass spectrometer for measuring the mass of ions and, more particularly, to a mass spectrometer of the magnetic field type for analyzing the mass of ions by causing an ion beam to pass through a narrow slit and then through a deflecting magnetic field to be detected by an ion detector where the intensity of the deflecting magnetic field is varied.
2. Description of the Prior Art
In a mass spectrometer of the magnetic field type, if it is assumed that the slit width of an ion source is "s", the slit width of an ion detector is "d", the mass dispersion coefficient is A.sub..gamma., the image magnification is A.sub.x, and the spread of the image by aberration is .DELTA., the resolution R of the mass spectrometer is generally expressed by the equation: ##EQU1## The relationship d=A.sub.x s+.DELTA. should exist in order to detect ions at a higher efficiency. In order to improve the resolution R, values have to be determined for the constants on the right side of equation (1) so as to set the numerator at a larger value and the denominator at a smaller value. If the slit width s is made narrower, however, the volume of ions extracted from the ion source is reduced, thereby lowering the sensitivity. It is desirable, therefore, to reduce the image magnification A.sub.x and, at the same time, to lower the aberration .DELTA. to attain a smaller denominator.
The ion beam emitted from the slit of the ion source is made to pass through a deflecting magnetic field. As the ion beam advances, it diverges to widen the width thereof whereas when the mass of the ion to be analyzed is large, the magnetic field has to be intensified and the gap spacing between the magnetic pole pieces has to be made smaller. In conventional devices, an ion beam may be set at 5 to 15 mm in width in a direction vertical to the median plane (the y direction), but the gap spacing between the magnetic pole pieces must be narrower than that range in order to measure the mass of high molecular compounds of a molecular weight as high as several thousand. As a result, an ion beam has only been partially utilized in the prior art.